This invention relates to an adaptive torque control of a cutoff knife pull roll. The invention is concerned with traveling webs of continuous sheet material, such as paperboard or the like, and concepts involving the handling thereof.
During normal operation of known web handling equipment, such as a corrugator or the like, a continuous web of sheet material is fed to a cutoff knife which severs the web transversely to form rectangular sheets of uniform length which are then fed downstream for further processing. A measuring wheel assembly is disposed upstream of the knife and includes a wheel which rides on the web, with the wheel shaft driving a position encoder which creates a signal which in turn actuates the knife for cutoff.
In some instances, it is desired to slit the traveling web longitudinally into a plurality of separate narrower side-by-side web segments. This is accomplished by a slitting machine which is disposed upstream of the cutoff knife. The slitting machine may be of the type disclosed in U.S. Pat. No. 4,627,214 issued Dec. 9, 1986, and includes a plurality of transversely spaced slitting tools which may be adjusted transversely to vary the width of the slit web segments. This adjustment can only be made when no web is present in the machine.
If it is desired to adjust the slitting tools without completely stopping the run of web, with its undesirable machine downtime, it is necessary to form a gap in the web. Previously known devices have formed such a gap by providing a shear upstream of the slitting machine and measuring wheel assembly. The shear is used to transversely sever the traveling web to form a separate downstream "tail" portion which is then accelerated relative to the web portion upstream of the sever. The resultant gap then allows transverse adjustment of the slitter tooling.
Immediately after shearing, the measuring wheel assembly is no longer effective in measuring the web for cutoff purposes because the measuring wheel is not being driven by the tail. In order to continue accurate cutoff length of the tail, measurement of the web is transferred from the measuring wheel assembly to a pull roll assembly located adjacent and just upstream of the cutoff knife. The pull roll assembly has usually consisted of a motively driven pull roll disposed beneath the web, and holddown idler roll wheels disposed above the web which biases the web downwardly against the pull roll. A position encoder is driven by the pull roll for purposes of taking over the signaling duties from the measuring wheel encoder.
In transferring the signal output from the measuring wheel encoder to the pull roll encoder after shearing, it is essential that their outputs be properly equated. In known devices, this has been accomplished by calibrating the pull roll with respect to the measuring wheel at a time when there is continuous web flow of equal velocity passing both measuring devices. A computer control has been utilized to accomplish the desired equating by applying a suitable multiplication factor to the pull roll encoder.
It is furthermore essential that, during calibration, no slipping occurs at the interface between the pull roll and the web. Heretofore, attempts have been made to eliminate slip by manually limiting the maximum torque output of the motive means, hereinafter referred to as a motor, driving the pull roll. However, problems occur in that various operating conditions may nevertheless cause slip. Such conditions may include the desire to pull a heavy web into and through the cutoff knife, and yet not slip on a very narrow web. Furthermore, the coefficient of friction between the web and pull roll can vary widely and cause slippage whenever the coefficient is low.
The concepts of the present invention are directed to assuring that the pull roll assembly does in fact accurately measure web travel after the web has been severed at the upstream shear.
In accordance with the various aspects of the invention, a control system is established wherein, when there is a flow of continuous web, an identification phase is provided by increasing the torque signal to the pull roll motor until slippage between the pull roll and continuous web is created at a breakaway torque point, the value of which is measurable. The pull roll motor is then caused to be driven by a signal which creates a motor torque output which is of a value less than the breakaway torque point. Thus, no slippage can occur. As the motor is driven at the lower torque value, a calibration phase is entered wherein the encoder output of the pull roll is properly equated with that of the measuring wheel, this phase occurring at a time when there can be no slip between the web-roll interface. The identification-calibration process can be repeated as often as desired.
The control system further includes the ability to cause acceleration of the pull roll after the web has been severed upstream to thereby accelerate the tail or downstream portion of the severed discontinuous web until the desired gap is created for use in a slitting device, as described earlier. At this point, which is subsequent to the identification-calibration phases, the control returns the motor input to the lower torque value.